Owing to the recent technical advancements of image processing by a computer, storage of a large amount of data and data communication, input of information, revision, edition, layout, and pagination are consistently computerized, and electronic editorial system enabling instantaneous output on a remote terminal plotter through a high speed communication network or a communications satellite has been practically used.
Light-sensitive materials having high photosensitivity which may provide direct type printing plate precursors directly preparing printing plates based on the output from a terminal plotter include electrophotographic light-sensitive materials.
In order to form a lithographic printing plate using an electrophotographic light-sensitive material, a method wherein after the formation of toner image by an electrophotographic process, non-image areas are subjected to oil-desensitization with an oil-desensitizing solution to obtain a lithographic printing plate, and a method wherein after the formation of toner image, a photoconductive layer is removed in non-image areas to obtain a lithographic printing plate are known.
However, in these methods, since the light-sensitive layer is subjected to treatment for rendering it hydrophilic to form hydrophilic non-image areas or removed by dissolving out it in the non-image areas to expose an underlying hydrophilic surface of support, there are various restrictions on the light-sensitive material, particularly a photoconductive compound and a binder resin employed in the photoconductive layer. Further, printing plates obtained have several problems on their image qualities or pinning durability.
In order to solve these problems there is proposed a method comprising providing a transfer layer composed of a thermoplastic resin capable of being removed upon a chemical reaction treatment on a surface of an electrophotographic light-sensitive element, forming a toner image on the transfer layer by a conventional electrophotographic process using a liquid developer, transferring the toner image together with the transfer layer onto a receiving material capable of forming a hydrophilic surface suitable for a lithographic printing, and removing the transfer layer to leave the toner image on the receiving material whereby a lithographic printing plate is prepared as described in WO 93/16418.
According to the method for preparation of printing plate using the transfer layer, a printing plate having good image qualities is obtained without various restrictions on the photoconductive layer employed in the hitherto known methods as described above. However, in the above-described method, the step for removing the transfer layer by a chemical treatment is indispensable in order to prepare a printing plate. Thus, simplification on a process for the preparation of printing plate and improvement in working efficiency are still limited.
Also, a method comprising development of an electrostatic latent image on an electrophotographic light-sensitive element with a liquid developer and transfer of the toner image thus formed onto a receiving material such as paper has been known. Such a method, however, causes specific problems based on the liquid developer as described, for example, in Eiichi Inoue (supervised), Shashin Kogaku no Kiso-Higin-enyou Shashin Hen, p. 364, Nippon Shashin Gakkai (ed.), Corona (1982). Specifically, when the development is conducted using a liquid developer containing toner grains having a diameter of from 0.2 to 2 .mu.m, a layer of carrier liquid having a thickness of from 100 to 200 .mu.m is formed on the surface of electrophotographic light-sensitive element, and when the developed light-sensitive element is brought into contact with a receiving material to transfer, the toner image does not sufficiently adhere to the receiving material due to the presence of carrier liquid, resulting in decrease in transfer efficiency. On the other hand, when the carrier liquid is removed, adhesion force between the toner image and the surface of light-sensitive element becomes greater and the transfer efficiency of toner image tends to deteriorate. Therefore, decrease in image qualities on the transferred image cannot be avoided.
Various investigations on improving a liquid developer used and a transfer method have been made in order to solve the above-described problems.
One method comprising, after development of an electrostatic latent image on an electrophotographic light-sensitive element with a liquid developer, transferring the toner image thus formed to a receiving material by contacting together the surfaces of the electrophotographic light-sensitive element and the receiving material while maintaining a large amount of the carrier liquid under applying a transfer potential using a corona charger or a bias roller has been proposed. However, in this method the carrier liquid present between the light-sensitive element and the receiving material may flow during transfer to cause streaks in the transferred image.
In order to prevent such a defect, JP-B-51-44654 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method wherein a thickness of the carrier liquid on the light-sensitive element after development is controlled in a range of from 5 to 30 .mu.m by corona discharge before the transfer of toner image. This method eliminates the above described problem by remaining the necessary and sufficient amount of carrier liquid for wet type transfer between the light-sensitive element and the receiving material and attains the improvement in transfer efficiency. Also, unnecessary penetration of the carrier liquid into the receiving material is depressed.
Although the streaks in the transferred image due to the excess carrier liquid present during the transfer are prevented to a certain extent by controlling the thickness of carrier liquid according to this method, there is another problem in that the toner image formed on the light-sensitive element per se is disordered by the corona discharge employed for controlling the thickness of carrier liquid, which results in the formation of cutting, distortion or spread of image.
JP-A-1-225975 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method wherein an amount of the remaining carrier liquid is controlled by introducing a drying step after development and then the toner image is transferred by a pre-charge step with corona discharge. However, in order to obtain a transferred image having good image qualities in a stable manner, it is necessary to extremely precisely control a space between the light-sensitive element and the receiving material.
There is a method for transfer the toner image by directly contacting the light-sensitive element bearing the toner image with a receiving material using a roller. This method, however, accompanies the flow of carrier liquid on the light-sensitive element and thus, the disorder of toner image on the light-sensitive element and the streaks in the transferred image may occur.
In order to prevent such defects, a method wherein two transfer rollers are provided and the opposite bias voltage are applied to these rollers is described in JP-B-46-1799. This method is not satisfactory, although the disorder of toner image is prevented to a certain extent.
Further, a method of interposing a resinous solution between the light-sensitive element and the receiving material to uniformly control the amount of toner transferred is disclosed in JP-A-55-95971, and a method of using a liquid developer containing spacer particles for preventing collapse of toner grains during transfer is disclosed in JP-A-59-100458, JP-A-60-95550 and JP-A-62-14168. However, these methods are still insufficient in view of the transfer efficiency or the disorder of toner image. In addition, when the receiving material having the toner image is employed as a printing plate, the resin used adheres to the surface of printing plate which tends to cause background stains on prints.
Moreover, various methods for transferring the toner image on the light-sensitive element to a receiving material not directly but through an intermediate transfer medium have been investigated. For instance, methods wherein the toner image is transferred from the light-sensitive element to an intermediate transfer medium is conducted by electrostatic transfer using corona discharge as described above and transfer to a final receiving material is performed by contact transfer using pressure together with heating are described in JP-A-63-34573, JP-A-2-264280, JP-A-3-243973, JP-A-4-50968, JP-A-4-507303, JP-A-5-503166 and JP-A-6-508444. Even in these methods, the toner image is not completely transferred from the light-sensitive element and the light-sensitive element must be cleaned. Also, it is difficult to fully dissolve the disorder of transferred image during the contact transfer with heating. Thus, these methods are yet insufficient for the formation of highly accurate image which is particularly required nowadays.
In order to solve various problems accompanied by the transfer to a final receiving material, JP-A-5-100579 discloses a method using two intermediate transfer media wherein the second intermediate transfer medium has an adhesive or adherent surface and transfer of the toner image to the first intermediate transfer medium is carried out electrostatically, and after drying the carrier liquid, the toner image is transferred by contact transfer to a final receiving material via the second intermediate transfer medium. However, since the method includes the increased number of transfer times, it is necessary to control each step with accuracy for obtaining the image having good image qualities in a stable manner. Further, construction of a device for transfer is complicated and it is difficult to assemble a simple and compact platemaking machine.
On the other hand, a method for transferring a conductive dry toner via an intermediate transfer medium having an adhesive surface is described, for example, in Tadashi Ogasawara and Masatoshi Kimura, Daiyonkai Non-Impact Printing Gijutsu Symposium Ronbunshu, p. 101 (1987). When the method is carried out using a liquid developer, transfer failure occurs due to decrease in the adhesion force of surface of intermediate transfer medium resulting from the contact with the carrier liquid, or transfer failure or disorder of tone image tends to occur during transfer to a final receiving material. Therefore, the specific problems based on a liquid developer has not been dissolved.